Early life pet exposure has been associated with lowered risk of allergic disorders. However, the mechanism underlying this association is unknown. The objective of this research is to determine how dog-keeping impacts early life immune development and whether this protection is associated with the bacterial profile of an infant's intestinal tract. Our hypothesis is that dog-keeping alters the bacterial community composition (BCC) in the home and, in turn, impacts the establishment of an infant's intestinal bacterial content (microbiota). Furthermore, dog-keeping, most likely through altering the intestinal microbiota, affects early childhood immune development in a manner that explains the observed lower risk of allergy. Our aims are: 1) To compare markers of immune development during the first 18 months of life among children living in homes that contain a dog versus those living in pet-free homes, 2) To compare characteristics of the intestinal BCC, as reflected by stool samples collected at 6 months of age, between infants living in homes that contain a dog versus those living in pet-free homes, and 3) To correlate characteristics of the intestinal microbiota to markers of immune development over the first 18 months of life. Methods: A cohort of neonates born into dog-containing (n=60) and pet-free homes (n=60) will be evaluated following maternal recruitment during pregnancy. Survey data on relevant health outcomes and longitudinal trajectory patterns of immune development will be analyzed and compared in the children from dog-containing versus pet-free homes using datapoints at birth, 6 and 18 months of life. Immune outcomes will include total IgE level, developmental and maturation markers on dendritic cells and regulatory T cells and cytokine secretion profile in response to mitogen, innate stimuli and IgE cross-linking antibody. At 18 months, immune responses to dust mite and tetanus antigen and specific IgE levels to common allergens will also be evaluated. The intestinal microbiota will be characterized by culture-independent analyses of stool samples at 6 months of age using the PhyloChip. Intestinal bacterial characteristics will be compared between dog-exposed infants and those from pet-free homes and analyzed for correlation to health outcomes and patterns of immune development.